Cultures—such as starter cultures—are used extensively in the food industry in the manufacture of fermented products including milk products (such as yoghurt, butter and cheese), meat products, bakery products, wine and vegetable products. The preparation of cultures is labour intensive, occupying much space and equipment, and there is a considerable risk of contamination with spoilage bacteria and/or phages during the step of propagation. The failure of bacterial cultures by bacteriophage (phage) infection and multiplication is a major problem with the industrial use of bacterial cultures. There are many different types of phages with varying mechanisms to attack bacteria. Moreover, new strains of bacteriophages appear.
Strategies used in industry to minimise bacteriophage infection, and thus failure of a bacterial culture, include the use of: (i) mixed starter cultures; and (ii) the alternate use of strains having different phage susceptibility profiles (strain rotation).
(i) Traditionally, starter cultures in the dairy industry are mixtures of lactic acid bacterial strains. The complex composition of mixed starter cultures ensures that a certain level of resistance to phage attack is present. However, repeated sub-culturing of mixed strain cultures leads to unpredictable changes in the distribution of individual strains and eventually undesired strain dominance. This in turn may lead to increased susceptibility to phage attack and risk of fermentation failures.
(ii) The rotation of selected bacterial strains which are sensitive to different phages is another approach to limit phage development. However, it is difficult and cumbersome to identify and select a sufficient number of strains having different phage type profiles to provide an efficient and reliable rotation program. In addition, the continuous use of strains requires careful monitoring for new infectious phages and the need to quickly substitute a strain which is infected by the new bacteriophage by a resistant strain. In manufacturing plants where large quantities of bulk starter cultures are made ahead of time, such a quick response is usually not possible.
Several attempts have been made to improve the resistance of cultures for use in industry.
Pedersen et at (7th symposium on lactic acid bacteria: genetics, metabolism and applications, Sep. 1-5, 2002, Egmond aan Zee, The Netherlands) teach a phage resistant Lactococcus lactis strain, which has no thymidylate synthase activity and which requires thymidine for DNA replication.
WO 01/14520 discloses a lactic acid bacterium which have a reduced susceptibility towards attack by at least one type of bacteriophage. Said lactic acid bacteria comprise a mutated gene involved in pyrimidine metabolism, namely pyrG which results in a defect in CTP-synthetase.
Kosuge et al (1998—Appl. Environ. Microbiol., Volume: 64, Issue: 11, Page(s): 4328-4332) and Kosuge et al (1994—FEMS Microbiology Letters, 123 (1/2) 55-62) teach a Thermus thermophilus HB27 bacterium which is mutated in the proB gene and is unable to utilise proline for growth.
However, there is a continuing need to improve cultures for use in industry.